1. Field of the Invention
The present invention relates to a backlight module, and in particular to a backlight for a liquid crystal display merging light emitted from light sources of different colors.
2. Description of the Related Art
Liquid crystal displays (LCDs) are applied in many electronic devices, such as cellular telephones, digital cameras, and personal digital assistant, and other devices used in environments of width varying degrees of light and shade degrees environmental brightness. In order to provide adequate display, backlight sources are located in the rear of the panels. In conventional liquid crystal displays, cold cathode fluorescent tubes (CCFLs) emitting white light serve as the backlight sources. However, because it is difficult to manufacture CCFLs with small diameters, the CCFLs are limited in application to small electronic devices. In addition, the CCFLs have higher voltage requirements and larger power consumption, are susceptible to electromagnetic interference, and present environmental concern due to use of mercury wherein.
Accordingly, light emitting diodes (LEDs) have been applied as backlight sources for LCD. White LEDs can serve as a backlight source, but have high manufacturing cost, or red, green, and blue LEDs can serve as source by merging three colors into white light, requiring additional optical merging components.
A backlight system is described by Yourii Martynov et al, “high-efficiency slim LED backlight system with mixing light guide”, 2003 SID Digest pp. 1259–1261 as shown in FIG. 1 in a schematic cross-section. The LED backlight system comprises a plurality of LEDs, a 90° elliptical mirror 10, a mixing light guide 11, a 180° elliptical mirror 12, and a main light guide 13. The LEDs are disposed in a row and comprise red, green, and blue LEDs, although only LED D11 is present in FIG. 1. Red, green, and blue light from the LEDs is reflected by the 90° elliptical mirror 10 to the mixing light guide 11, wherein the direction of light is changed by reflective layers of the mixing light guide 11. White light is produced by continuous changing of the direction of the red, green, and blue light. The mixed white light exits the mixing light guide 11 and is then reflected by the 180° elliptical mirror 12 to the main light guide 13. The main light guide 13 converts the mixed white light to planar light and emits the planar light to back-end optical components, for application to the LCD panel.
However, since the direction of the light is changed only when contacting the reflective layers, adequate merging length is required in the mixing light guide 11. In the above LED backlight system, however the merging distance is around 75 mm, a distance which cam limit application in small device.